1. Field of the Invention
The invention relates to the field of forming doped regions in a semiconductor material such as in the formation of source and drain regions for field-effect transistors.
2. Prior Art
Often in the formation of source and drain regions for field-effect transistors, a dopant is implanted into the substrate in alignment with a gate such as a polycrystalline silicon (polysilicon) gate with or without spacers. The dopant is then driven into the substrate in an annealing step. Most often the ion implantation occurs through an oxide layer although in some cases the dopant is implanted directly into a silicon substrate.
There are advantages to implanting the dopant through an oxide layer. First, if the dopant is covered by the oxide layer in the drive step it is less likely to evaporate during the drive step. Secondly, the oxide layer provides protection from impurities that might otherwise collect on the substrate both during pre and post implant steps. There are, however, disadvantages to implanting dopant through the oxide. First, oxygen from the oxide layer is driven into the substrate during the implantation. This can cause leaky junctions to occur. Additionally, the oxygen once in the substrate, makes it more difficult to form a salicide once the oxide has been removed and a metal layer is formed over the source and drain regions.
As will be seen the present invention provides at least some of the advantages obtained from ion implanting through an oxide without the disadvantages which occur when the implantation occurs through the oxide layer.